Method of forming malleableized iron castings



June 18, 1957 c. BERG 2,796,373

METHOD OF FORMING MALLEABLEIZED IRON CASTINGS Filed Feb. 5, 1954 l4Sheets-Sheet 1 9 P" 51; mm P51 Kg per sq. mm I P 100 0oo' HB (S /3 60060 20 5aooo HB Q5 200 20 I I I I I 0 100 500. v 600 100 800 900 1000 F 0I 100 000 500 'c IT g1 INVENTOR.

June 18, 1957 c. BERG 2,796,373

METHOD OF FORMING MALLEABLEIZED IRON CASTINGS Filed Feb. 5, 1954 14Sheets-Sheet 5 K9 er sq.mm P51 150.ooo 100 -IOQOOO 2 3 H- 5 5 7 8 910 1520 Charge No.

INVENTOR;

c. BERG 2,796,373

METHOD OF FORMING MALLEABLEIZED IRON CASTINGS June 18, 1957 14Sheets-Sheet 4 Filed Feb. 5, 1954 O 0 0 m w w w o w M w w M 2 2 H w W. Mw m w o B 9 1, a 5 0 EM M E l 6, \c) I i 7 t I I x x u M I I I I m .17m, IL 5 V w l V r F I I c 81 0 mm .l I, 6 4 IN- l l li O R Mf/ 07 WW mam, M 0 0 O 0 O 0 O O 0 0 O m m m m w w m w w June 18', 1957 c. BERG2,796,373

METHOD OF FORMING MALLEABLEIZED IRON CASTINGS Filed Feb. 5, 1954 14Slieets-Sheet 5 Fig. 10. White cast iron with decomposed austenice ands. cementite net Fig. 11. Same as annealed. 650-200 temper carbonparticles pro mm lizzventor June 18, 1957 c. BERG 25796373 METHOD OFFORMING MALLEABLEIZED IRON CASTINGS Filed Feb 5, 1954 '14 Sheets-Sheete;

Fig. 12. Hardened 8 min/900 C/quenched. in

water 500 x Fig. 15. Same tem aged 1+0 min/1400 0 12 kg m 1.7% elong.,565 Brinell Inc: enter June 18, 1957 c. BERG METHOD OF FORMINGMALLEABLEIZED IRON CASTINGS 14 Sheets-Sheet 7 Filed Feb Fig. Hardened l8min/9OO C/quenched in water nell "0 ;0% 810118., 588 Bra].

Fig. 15. Same, tempered 11.0 min/L 00 126.5 kg/mm June 18, 1957 c. BERG2,796,373

METHOD OF FORMING MALLEABLEIZED IRON CASTINGS Filed Feb. 5, 1954 14Sheets-Sheet 8 Fig 16. Hardened 25 min/900C/quenched in water 500 X Fig.17. Same, tempered lg.0 min/14.00 6

12L- .5 kg/mm LM e 0ng., 588 Brinell 500 x C. fiery June 18, 1957 c.BERG METHOD OF FORMING MALLEABLEIZED IRON CASTINGS Filed Feb. 5, 1954 14Sheets-Sheet 9 Fig. 18. Hardened 115 min/9OO C/quenched in water 500 xFig. 19. Same, tempeI 'e d 110 min/1100 0 117.6 kg/mm 1.5% 610113., 588Brinell 500 x .Zizzv elztaz" June18, 1957 c. BERG 2,796,373

METHOD OF FORMING MALLEABLEIZED IRON CASTINGS Filed Feb. 5, 1954 14Sheets-Sheet l0 Fig. 20. Hardened 120 min/900 C/quenched in water Fig.21. Same, tempered ho min/)+0OC 118.9 kg/mm 1.5% elong., L 15 Brinell500 x June 18, 1957 CQBERG 2,796,373

METHOD OF FORMING MALLEABLEIZED IRON CASTINGS Filed Feb. 5, 1954 14Sheets-Sheet l1 Fig. 22. Austempring. 1' min/900C 1o h/250c 110 kg/mm 11.71 elong., 14.15 Brinell Fig. 25. Axistempring. l7 min/900C 50 min/510C112.9 kg/mm 1.5% along. 565 Brinell 500 x Ire/v avatar C .15 81 J1me1957 c. BERG 2,796,373

METHOD OF FORMING MALLEABLEIZED IRON CASTINGS Filed Feb. 5, 1954 14Sheets-Sheet 12 Fig. 2 Austempring. 17 min/900C. 5 min/11.00 0

99.5 kg/mm 1.7% along. 521 Brinell Fig. 25. 0.126% P. Intercrystallinefissures 61.6 kg/mm 1.0% along. 588 Brinell 500 x liz'zvezataz June 18,1957 c. BERG 2,796,373

METHOD OF FORMING MALLEABLEIZED IRON CASTINGS- Filed Feb. 5, 1954 I 14Sheets-Sheet 13 Fig. 26. 0.126% P. Severe cracks.

22 kg/mm o,9% elong., 588 Brineg Fig. 27. 0.28% Mn. Coarse structurewith crack 95.2 kg/mm 1.0% elong., 588 Brinell' June 18, 1957 c. BERG 2,3

METHOD OF FORMING MALLEABLEIZED IRON CASTINGS Filed Feb. 5, 1954 14Sheets-Sheet 14 Fig. 28-Spher0ida1 structure, annealed 55.5 kg/mm 16.3%el0ng., 125 Brinell 300 x Fig. 29. Same heat treated. Spheroid andtemper carbon. 18 min/900C L 0 min/14.00 0

125.9 kg/mm 2.0% e10ng., 592 Brine'll 500 x United States atentQ METHODOF FDRMING MALLEABLEIZED IRON CASTINGS Claes Berg, Overum, Sweden,assignor to Aktiebolaget Overnms Bruk, Over-um, Sweden ApplicationFebruary 5, 1954, Serial No. 408,548

Claims. (Cl. 148--21.7)

The present invention broadly relates to the art of metal treatment.

The invention particularly relates to a method of producing ferrouscastings and the casting so produced.

This application is a continuation in part of my applications Serial No.23,858, filed April 28, 1948, now abandoned, entitled Manufacture ofMalleableized Iron Castings Particularly Wearing Parts of AgriculturalImplements, and Serial No. 384,313, filed October 5, 1953, and entitled,Heat Treated Malleable Cast Iron and Methods of Producing the Same.

More particularly the invention relates to the production of ferrouscastings having markedly increased physical properties as compared withknown castings, with particular reference to cast parts that aresubjected to wear under various abrasive conditions.

The invention therefore is particularly related to the manufacture offerrous castings including the heat treatment thereof and to the castingthus produced having considerably increased utility, wearability andtoughness.

It is therefore an object of this invention to provide an improvedmethod of producing ferrous castings and an improved casting of the typehaving a white fracture and to subject the casting to such heattreatment steps that the resultant product has a markedly increasedtensile strength as compared with known cast iron parts.

A further object is to so control the composition of the casting and theheat treatment steps that the resultant product has an exceptionallyhigh Wear resistance, particularly when subjected to conditions such asabrasion in soil, particularly of a rocky nature.

A further object is to provide a cast iron part broadly constituting aheat treated malleable iron that can be forged and rolled in a mannersimilar to ordinary steel parts and which, contrary to expectation, canbe welded electrically.

An additional object is to correlate the ingredients or componentcompositions of the cast iron with the various heat treating steps sothat production of the finished product can be controlled as touniformity and desired physical properties.

It is therefore a specific object of this invention to provide a methodof producing ferrous castings and the resultant cast product in which aparticular range of certain ingredients is incorporated in the castingswith the view of permitting the development of exceptionally hightensile strength and in which the heat treating steps include an initialannealing or malleabilization controlled as to the structure producedfrom this step, followed by a further heat treatment or hardening thatis controlled as to temperature and time after which a liquid quenchingis employed of sufiicient rapidity that the matrix of the structure is,as far as possible, free from fissures and micro cracks and is free ofsuch disadvantages that would affect the properties of the finishedcasting.

As a further specific aspect following the hardening or reheating of thecasting and the quenching thereof, this invention has an additionalobject to provide a tempering step that results in a cast product havingthe desired physical properties.

The prior art is replete with disclosures of the production of malleableiron cast products in which an iron casting that is termed to be more orless a white iron casting is annealed and then heat treated to obtaincarbon in the combined form. The tensile strength obtained in the priorart methods, .as far as is known, has a general maximum of 100,000 p. s.i. Consistent with this factor, the ASTM Standards Specification 1952Ferrous Metals designation A220-50 indicates that the minimum tensilerequirements for pearlitic malleable iron castings are between from60,000 to 90,000 p. s. i. tensile.

In accordance with the method fully set forth hereinafter, the presentinvention results in the production of a casting that is of suchimproved tensile strength that this factor has been increased to 200,000p. s. i. and somewhat above. Therefore the method of the presentinvention results in the production of castings, the tensile strength ofwhich is uniformly within the range of 120,000 to 200,000. The method isof such nature that the tensile strength of the finished product can bewell controlled within these ranges. It has been found that in castingsproduced according to the invention elongation and wear resistanceusually increases, if the heat treatment is performed in such a way thata maximum tensile strength is obtained.

Therefore in accomplishing the present invention, a casting is producedwhich has a substantially completely white structure as revealed bymicroscopic examination of a polished surface and which structure,insofar as is possible, is marked by an absence of flake graphite. Inthis connection, the composition of the iron to be cast is controlledwithin specified limits and the metal contains controlled amounts ofcertain specific elements, the quantities of which, when the hightensile strength is produced, are critical.

Secondly, the annealing or initial heat treatment which includes acooling cycle must be effected in such fashion that upon a subsequenthardening treatment the fissures and micro cracks, if any are formed,are so minute so as not to adversely affect the properties of thehardened casting. In this connection, the cast product after annealingis practically free of free cementite.

Further objects and advantages of my invention will be readily apparentfrom the following description of the invention, which is more clearlyunderstood by reference to the accompanying drawings, in which:

Figure 1 is a diagram illustrating as an example the tensile strength asa function of the phosphorus content,

Figures 2 and 3 are diagrams illustrating preferred annealing cycles,

Figures 4 and 5 are diagrams illustrating reheating or hardening cycles,

Figure 6 is a diagram illustrating times and heat factors in a temperingstep,

Figure 7 is a diagram illustrating the tensile strength, hardness andelongation as a function of the heat applied during the tempering step,

Figure 8 is a diagram illustrating the uniformity in tensile strength offinished castings in plural and successive production cycles in thecontinuous production of ferrous parts,

Figure 9 is a diagram illustrating the stable and metastable systems andthe formation of and into austenitic structure,

Figure 10 is a microphotograph illustrating the structure of an iron Aas cast to a white iron,

Figure 11 illustrates the structure of iron A as annealed in accordancewith the 'cycle of Figure 3,

Figures 12, 14, 16, 18 and 20 illustrate the structure of iron A afterreheating for hardening to 900 C. hold- 3 ing at this temperaturerespectively 8, 18, 25, 45 and 120 minutes, and quenching in water,

Figures 13, 15, 17, 19 and 21 correspond each to the preceding figureand illustrate the structures after tempering 40 minutes at 400 C,

Figures 22, 23 and 24 illustrate the structure of the iron A annealed inaccordance with Figure 11, reheated for hardening at 900 C. during 17minutes, and quenched in a bath at 250 C. during hours, 310 C. during 50minutes and 400 C. during 3 minutes, respectively,

Figures 25 and 26 illustrate cracks in the structure of an iron with0.126% phosphorus hardened and tempered as in Figures 14 and 15,

Figure 27 illustrates the structure of a similar iron as Figures 25 and26 but with a normal amount of phosphorus and an amount of manganeseincreased to 0.28%,

Figure 28 illustrates the structure of an iron as annealed andcontaining magnesium, and Figure 29 the same iron after hardening andtempering' In performing the present invention, 1 have established thata pre-requisite for obtaining the desired and markedly improved physicalproperties, particularly tensile strength and resistance to wear and foraccomplishing the other objects of the invention, the material ashardened shall be as free as possible from flake graphite, freecementite, and micro cracks. Further, that the material be such thatwhen hardened and quenched in water the matrix will consistsubstantially only of martensite. Therefore the cast iron composition asa starting material consists preferably of pig iron or other iron basematerials that contain only small amounts of elements that affect theproperties of the finished product. Good results in casting have beenobtained by utilizing a rotary or stationary air or electric furnacealthough the type of melting unit is not considered to be a restrictionof the present invention. Casting conditions should be such, thatcoupled with the composition of the melt, a casting is obtained that isas free as possible from graphite. Thus when casting in dry-sand moulds,it will be advisable to take the castings out of the moulds immediatelyafter solidification in order to promote a rapid cooling.

In the production of ordinary malleable iron castings the composition ofthe metal as cast may vary within rather wide limits with respect tocarbon, silicon, manganese, phosphorus and other alloy elements, withoutessentially impairing the properties desired in the malleable iron assuch. In connection with my invention, however, and to obtain a finishedproduct having the properties in accordance therewith it is necessarythat the composition of the metal as cast be controlled within narrowlimits as to the specific amounts of several elements present.

The total amount of carbon and silicon should be sufficiently low toallow freezing of the melt to a white iron. It is not suificient toproduce a semi-white iron or partly mottled iron. Such condition can beindicated by the fracture or by observation of a section under themicroscope. The casting must be entirely white and free from any form offlake graphite. Therefore the carbon content and the silicon content canprincipally vary within the limits shown in known diagrams for whiteiron, such as the diagram of Maurer, but the carbon content is usuallylimited to between 1.83.2% and the silicon content adversely to betweenl.60.5%. The best results have been obtained with carbon between 2.22.4%and silicon about 1%. The total amount of carbon and silicon ispreferably between 3.4 and 3.8%.

In the hardened product according to the invention, sulphur, especiallycombined as iron sulphide, is an impairment in the material since ironcarbide appears to dissolve some iron sulphide which is therebystabilized and more difiicult to decompose by annealing. If the sulphurcontent is not sufficiently low, cementite will remain in the hardenedstructure thereby inducing brittle characteristics reducing theductility as measured by elongation percent and causing the promotion ofstrains which can result in the formation of micro cracks which arereadily revealed in the polished sections etched and submitted tomicroscopic examinations. The sulphur in the castings should thereforebe very low and definitely less than 0.1% combined with iron. It ispreferred that the total sulphur content be less than 0.05%. Irons arenow available having a sulphur content as low as 0.01%. Therefore it ispreferred that the sulphur content be between 0.01 and 0.09% with thebest results following the reduced sulphur content within this range.

While manganese to a certain extent is necessary to hold the sulphur inchemical combination, the manganese also stabilizes the austenite andcementite by forming complex carbides. In the successful performance ofthe present invention, the presence of a high manganese content is to beavoided since this element impairs the properties of the hardenedproduct in a manner similar to sulphur. It has especially been observedthat a high manganese content causes intercrystalline hardening cracksif the iron is heated for hardening above about 925 C., and in case of arelatively high manganese content a temperature of about 900 C. istherefore preferred. However, it should be pointed out that a highmanganese content also reduces the possibility of welding the material.Tests have been conducted on materials manufactured and processedaccording to the present invention with variations in the manganesecontent in instances where the other ingredients of the casting areWithin the ranges set forth on the table hereinafter disclosed andreveal that an increase of the manganese content lowered the tensilestrength of the finished product. Therefore manganese content should bekept low. However, the content of the metal should not be too low sincemanganese is required to assure the complete solution of ferrite duringthe reheating of the casting. Therefore, there should be a minimum of0.05% and a maximum of 0.45% manganese, and in any case the manganesedissolved in iron should be below 0.20%. The total manganese contentusually varies between 0.1 to 0.3%, and the preferred range of manganeseis between 0.18 and 0.20%.

The phosphorus content is of a critical nature when the end product musthave an extremely high tensile strength. Phosphorus has previously beenconsidered to have no influence on the graphitization, and in ordinarymalleable iron, as revealed in handbooks, a relatively high content ofbetween 0.10 to 0.20% and more is often used with the object in view ofincreasing the fluidity of the material as cast. I have found, however,that a high amount of phosphorus content causes the graphitizationduring annealing to follow more closely the curve corresponding to themetastable system rather than that of the stable system and thus some ofthe cementite content to resist decomposition. Thus a high phosphoruscontent will result in the formation of ferrite-graphite-cementiteeutectoid with grain boundary cementite around the austenite crystalsinstead of a ferrite-graphite eutectoid which is aimed at according tothe invention. When a material of a high phosphorus content is hardened,particularly by rapid quenching, the undecomposed cementite distributedin the form indicated is undesirable. Intercrystalline cracks are formedalong the boundary cementite, and the material becomes brittle, as a lowtensile strength, and cannot be welded or forged. Tests have beenconducted on material prepared in accordance with the table of rangesset forth hereinafter with varying phosphorus content and, as shown inFigure 1, the tensile strength is definitely a function of thephosphorus content. In this figure it is revealed that tensile strengthof 200,000 p. s. i. is attained with a phosphorus content of less than0.05%, usually 0.04%. When the phosphorus content was increased to 0.10%and 0.15% respectively, the tensile strength decreased to 120,000 p.- s.i. and approximately 82,000 p. s. i. respectively. The tests have alsorevealed that the increase of phosphorus retards the formation ofaustenite when the material is heated for hardening, and in order toavoid undissolved ferrite it is necessary, either to extend the holdingtime with the disadvantage that some austenite will be too saturated andthe hardening cannot be controlled, or to raise the hardeningtemperature. In both cases the hardened iron will be further impaired.

It is therefore of great importance to assure a low phosphorus contentwith less than about 0.10% or 0.09% and preferably less than 0.05%. Asstated, the best results have been obtained with less than 0.04%, suchas by the use of a charcoal iron having a phosphorus content of about0.03%.

I further want to point out that the improved properties of the productresulting from following the teachings of my invention do not requirethe use of additional alloying elements such as chromium, nickel,molybdenum and the like. In certain instances such additions havedeleterious effects. Thus chromium will retard graphitization and nickelwill reduce the solubility of carbon in iron. However, within theframework of the present invention I do not exclude for certain specialpurposes the addition of small amounts of alloying elements such ascopper to improve corrosion resistance or aluminum or titanium whichfavorablyinfluence elongation and can be used with advantage up to about0.1%.

Therefore within the teachings of the present invention the chemicalcompositions of the casting after hardening will include in addition toiron the following elements in the stated ranges: carbon 1.8-3.2% andpreferably between 2.0-2.6%, silicon 0.51.6% and preferably about 1%,manganese ODS-0.45% and less than 0.20% combined with iron, preferably atotal conw been added, and the properties of I the finished product tentof 0.18-0.20%, sulphur less than 0.10% combined with iron, preferablyless than 0.05%, phosphorus less than 0.10% and preferably about 0.04%and the remainder substantially iron.

The iron alloy described above can be cast to a completely white irononly if the maximum section size of the casting is less than about ormm. In heavier castings more or less flake graphite will appear, whichwould make the material unusable for further heat treatment according tothe invention. This disadvantage can be avoided, if a metal of the groupmagnesium, cerium, calcium and lithium is added to the melt, as thesemetals have the capacity of preventing the segregation of flakegraphite. Of these metals magnesium is preferred. It melts at 657 C. andevaporates at 1102 C., which is below the melting point of iron, and itis therefore recommended to add the magnesium alloyed with other metals,such as Si and'Fe. In order to increase the specific gravity above thatof iron to cause sinking of the alloy in the iron melt a further andheavy metal may be used in the alloy, such as copper. A preferred alloyis thus 12-22% Mg, 25-35% Si, 40-50% Fe and 10-15% Cu. Some of themagnesium added to the melt will react with the air and be consumed byburning, and an other portion thereof will react with S to MgS. Theremaining amount of Mg should be more than 0.02%, preferably 0.03-0.09%,and particularly about 0.05% is recommended. Similar amounts of cerium,calcium or lithium can be used instead of magnesium and in the form of asuitable alloy. Such alloys can be added either as a melt or as smallpieces to the melted iron shortly before casting, and in order to securethat as great part as possible of the metal remains in the iron themetal can be introduced into the melt at a low level by being blown intothe same by means of compressed air. An example of such an iron alloyis: 2.5% C, 1% Si, 0.15% Mn, 0.04% S, 0.04% P, net 0.05% Mg or Ce, andthe rest substantially iron.

A further advantage of the addition of Mg or one of the other specialmetals is that a great number of inoculation points are obtained for thesubsequent formation of graphite in the form of rounded, denseparticles, sometimes called nodules, and more fluffy temper carbonnests. Most of these carbonparticles are formed during the subsequentannealing operation, but under certain conditions and withoutinconvenience a small amount thereof may appear in the material alreadyas cast.

The more compact and substantially spheroidal nodules are in this wayobtained instead of flake graphite and do not detrimentally affect theproperties of the finished material, as flake graphite would do. Incontrary, the properties are improved compared with the case that onlytemper carbon had been obtained as they have a less volume and moredesired form. On the other hand this compact carbon can only with greatdiificulty be dissolved and diffuse into the ferrite to form austeniteduring the hardening operation. For this reason a sufficient portion ofthe free carbon should be present as temper carbon, which dissolves veryeasily into austenite. It is usually sufiicient that 50% or more of thefree carbon occurs as temper carbon, but 60-80% is preferred. Thedesired proportion between the two kinds of carbon can be attained byvarying the amounts of Mg, Ce, Mn and other elements in the iron alloy,and thereby also the desired matrix of the finished product may bevaried. Thus it is possible by increasing the proportion between tempercarbon and the compact particles to get a material with a higher contentof combined carbon in the hardened castings for such articles as wearparts, which should have a great tensile strength and a high wearresistance. By lowering the said proportion the content of combinedcarbon is reduced and the elongation is increased, and such a materialis of special advantage for construction parts. This iron alloy is to beheat treated in the same way as if magnesium, or the like had not arealso substantially unchanged.

For the purpose of ascertaining the dilferent steps of the invention,illustrating the structures by photographs and testing the finishedproduct with respect to its mechanical properties a great number of testbars were cast which after heat treatment had the following chemicacomposition:

2.38% C, 0.89% Si, 0.19% Mn, 0.037% P, 0.031% S and the restsubstantially iron. This composition is hereinafter called Iron A.

The Iron A was cast, and immediately after solidification of thecastings the sand was removed and the castings were allowed to cool inair. The structure of the casting is shown in Figure 10 and comprisesdecomposed austenite crystals in a net of cementite without any flakegraphite. The magnitude of this and all other photographs is 300. Thetensile strength was 46,000 p. s. i. and the hardness 415 Brinell.

The iron was then annealed according to the cycle shown in Figure 3, inwhich the curve illustrates the temperature within the oven. The castingwas loosely placed in closed pots containing some charcoal to bind theoxygen in the air and prevent decarbonization of the iron. To obtain adistribution of temper carbon favorable to the subsequent hardening andto efiect decomposition of practically all cementite it is of importanceto anneal the material at a temperature considerably higher than 800 C.Preferably the annealing temperature should lie between 850-1050 C. orabout 950 C. The temperature of annealing must be held sufliciently longto convert all cementite into austenite and temper carbon but not heldto such an extent asto materially reduce the carbon content by surfacedecarbonization. On the other hand since elements such as sulphur andmanganese retarding conversion are present only in small amounts, therequired holding time for complete conversion will be relatively short.

7 shown in Figure 3, in which the curve illustrates the ternperaturewithin the oven. The temperature was raised to 350 C. in 2 /2 hours,maintained at this temperature for 10 hours, raised to 960 C. in 2 /2hours, maintained constant for 25 hours, lowered slowly to 750 C. in 4hours and thereafter more slowly to 650 C. in 25 hours, whereafter theoven is allowed to cool.

In order to make clear the transformation steps during the annealingprocess reference is made to Figure 9. The point E represents themelting point 1145 C. (1.7% C) of the iron in the iron-carbon ormetastable system and the point So the corresponding eutectoid point 721C. with 0.86% C. If a steel containing maximum 1.7% C completelysolidifies in a point on the curve represented by point E0 austenite isformed, and if the temperature is lowered, the carbon content of theaustcnite will dccrease according to the line E050 and the precipitatedcarbon will form cementite until the eutectoid or pearlite point So isreached. When this point is passed all austenite will be transformedinto ferrite and cementite forming pearlite.

ES represents the corresponding transformation line in the metastablesystem of an iron alloy containing carbon and 1% silicon which lineowing to the content of Si is displaced to the left. In point B thetemperature is 1160 C. and carbon content 1.6%, and the point S, wherethe temperature is 760 C. and carbon content 0.64%, represents the upperlimit of the pearlite range, about 20 C., within which at decreasingtemperature the austenite is transformed into pearlite. If at a certaintemperature the carbon content is higher than that shown by line ES, themetastable system can be transformed into the stable system E 8 Itshould be observed, however, that the exact form of these threetransformation lines is not fully known and that they may be slightlycurved, but in the present case it is thought to be sulficient to usestraight lines in the diagram.

If now the Iron A containing 2.38% C is heated to a temperature whichmay be 950 C., austenite is formed which dissolves cementite andprecipitates temper carbon, until the point U is reached, in which allcementite is dissolved. By further heating the austenite containing 1.1%C may yield further temper carbon, until point V in the stable system isattained and the combined carbon has decreased to 0.93%. On lowering thetemperature to 770 C. in point S the combined carbon content is 0.57%and thereafter in the pearlite range the austenite will be transformedeither to pearlite or to ferrite and temper carbon in dependence of therate of cooling. If this rate is as slow as shown in Figure 3 thestructure will contain substantially only ferrite and temper carbon. If,on the other hand, the iron should contain too much sulphur, phosphorus,manganese and other metals or if the holding time has been too short,some cementite will remain undecomposed and cause cracks on hardening.

In-order to establish the absence of. cementite, an examination of thetensile strength and elongation after the annealing treatment will beindicative of the absence of the cementite. A low tensile strength of upto about 50,000 p. s. i. in association with a high elongation of morethan about 16% will indicate a malleable iron which may be suitable forthe manufacture of a material of the invention. Usually such propertiesare not found in malleable iron manufactured for use in annealedcondition, or for further heat treatment as embodied in the prior art.

As shown in the diagram Figure 3 the temperature has been maintainedconstant at 350 C. for hours. The iron will thereby emit hydrogen to theeffect that the time of holding at 960 C. may be shortened at least l0hours and a more even distribution of a greater number of small tempercarbon is obtained. The anneal process, however, can be varied indifferent ways. According to the .diagramlshown in Figure 2 thetemperature of the casting is heated continuously to 960 C. in 20 hours,

maintained for 30 hours, lowered to 760 C. in v5 hours and to 670 C. in30 hours, whereafter the casting is removed from the oven and is cooledin the air. Also in this way a similar ferritic matrix is obtained. Thiscomplete anneal cycle can be interrupted in a point B between the annealtemperature 960 C. and the upper limit 770 C. of the pear-lite oreutectoid transformation range, preferably at about 850 C., by removingthe casting from the oven and allow it to cool in the air as indicatedby a dotted line. By Figure 9 it is revealed that the iron at 850 C.contains about 0.72% C combined in austenite and the rest of the totalcarbon content 2.38%, i. e. 1.66% C, as temper carbon. The austenitewill thereby be transformed into pearlite, and the proportion of theamounts of pearlite an'd ferrite can be regulated by the choice of asuitable temperature A.

The microstructure of Iron A annealed as shown in Figure 3 appears fromFigure 11. No cemcntite remains, and the temper carbon is welldistributed to a number of about 600 to 700 pro square mm.

The material can be reheated for hardening either from room temperatureor from any temperature below the pearlite range or below about 750 C.As the material has to be transferred from the anneal oven to thehardening oven, however, the temperature during this operation usuallywill decrease considerably below 750 C. On reheating, austenite will bereformed in point S from the ferrite and some temper carbon or pearlite.During the long anneal period the austenite grains have had sulficienttime to grow to a coarse structure, and hardening from a temperatureabove the pearlite range would lead to a coarse and brittle structure.The reformed austenite, in contrary, has very fine grains, and as theshort heating period for hardening is insufiicient to allow the crystalsto grow a very fine and desired structure after hardening is obtained.When the casting is heated up to a temperature of about 900 C. thecarbon content of the austenite varies principally as indicated by lineR in Figure 9, according to which the austenite will first slowly andthereafter more rapidly dissolve temper carbon and thereby increase itsoriginal carbon content 0.57%. On the temperature line 900 C. the pointrepresenting the carbon content moves to the right towards point V1 onthe line E 5 which represents the stable system. Now tests with thematerial to be hardened according to the invention have indicated thatthe austenite in this material can dissolve considerable amounts oftemper carbon up to substantially all temper carbon present and that thecarbon content probably may pass beyond the stable system. These testswill be discussed hereinbelow with reference to the photographs. Thecarbon content can thus pass the point V1 towards or beyond point U1 onthe metastable line ES. If the austenite is allowed to be fullysaturated or saturated to a carbon content above a certain value,quenching in a liquid will result in the formation of free cementite,which would make the material brittle, cause cracks and considerablyreduce the tensile strength and elongation. A maximum carbon content ofabout 0.8% has thus been found suitable, and preferably 0.65% to 0.70%is used. A simple method of controlling that the best carbon content isobtained is to control the heating and holding time. This control iseasier, if the time for heating the casting to the desired temperatureis as short as possible. The hardening oven is, therefore, preferablypreheated to the holding temperature or to 850-950 C. Since the annealedcasting following the complete anneal cycles of Figures 2 and 3 consistsof substantially only ferrite and temper carbon, and the castingfollowing the interrupted anneal cycle according to Figure 2 consists inaddition to ferrite and temper carbon of some formed pear-lite, rapidheating does not appreciably deform the casting as in case of ordinarysteel. If the teachings of this invention have been followed up to thecompletion of the annealing cycle the casting as reheated is initiallysubstantially stress free. As indicated the temperature range is wellabove the eutectoid or critical transformation range, and I haveascertained that the best results have been obtained by heating at atemperature about 900 C. I have further discovered that if thetemperature is raised above the range mentioned herein, namely 950 C.,the carbon content of the austenite increases so rapidly that thiscontent cannot be controlled. Castings of different section sizes heatedtogether will reach the holding temperature only successively and besubjected to such different holding times that a very uneven product isobtained with too much combined carbori in the small sized castings.Furthermore, when the material is thereafter quenched so as to promotethe formation of a martensitic structure, the martensite thus formedwill exhibit a coarse configuration of needle-like structure. The largeneedles thus formed will result in a final product constituting abrittle and unsatisfactory casting. If, on the other hand, the holdingtemperature is below 850 C., then it will be diflicult to ensure thatall ferrite present in the annealed matrix is transformed intoaustenite.

If the annealing cycle is interrupted in a point B in Figure 2, pearliteis formed, and upon reheating for hardening austenite will be reformedmore rapidly from the pearlite than from ferrite and temper carbon orspherolites. The holding time must in this case be still shorter than ifthe austenite is to be formed only by ferrite and free carbon, and thecontrol of the holding time is in this case of still greater importance.Also if the hardening is performed in combination with an isothermaltransformation it is of importance to control the holding time andespecially to obviate too long time.

Therefore, the material or casting should be kept at the hardeningtemperature, preferably between 875-925 C. or about 900 C.,substantially only for such period of time as will ensure that allferrite present has been transformed into reformed austenite. Therefore,I wish to point out that if the reheating period, particularly theholding time, is too long or such as the time customarily used inconnection with hardening steel, then the product will be unsuitable forthis invention.

The hardening heating cycles shown as an example in Figure 4 indicatethe temperatures of the castings placed in an oven preheated to aboutthe hardening temperature 900 C. The cycle C corresponds to castingshaving a maximum section size of up to about 25 mm., D between 25-50 mm.and E more than 50 mm. The heating times are shown to be about 6, l5 and35 minutes and the subsequent holding times 15, 20 and 30 minutesrespectively. The holding times may vary within certain limits independence of the composition and of the properties desired, butpreferably only within to 25 minutes in cycle C, to 30 minutes in cycleD and to 50 minutes in cycle E.

After the material or casting has been reheated, liquid quenching can beeffected in different ways. According to Figure 4 the casting ishardened by water quenching to a martensitic structure sufficientlyrapidly to avoid the formation of any essential amount of pearlite atleast in the outer layer of the casting, but if the casting is of aheavy size it is obvious that the interior thereof is cooled somewhatslower and may contain more or less pearlite which, however, is not ofany great disadvantage. The matrix will also be substantially free fromfree residual cementite assuming that the teachings of the inventionpertaining to the ingredients of the composition, the annealing cycleand the reheating cycle have been followed. Consequently, any crackshaving a deleterious effect on the end product are avoided. If some restaustenite should remain, the casting can be placed in cold water orcooled down to 85 C. for transformation into martensite. The structureof the material as quenched in water is shown in Figure 14.

If a greater elongation and toughness are desired, the casting may behardened by quenching rapidly in a bath to a temperature substantiallyabove the upper limit of the martensite range and below the pearliterange, preferably to between ZOO-450 C. If an efiective cooling issecured it is possible, owing to the special chemical composition, topass to the left of the S-curve, also known as the TTT-curve(Time-Temperature-Transformation), without the use of special metalalloy additions practiced heretofore, which would give rise to notdesired complex carbides, and it is thus possible to avoid any essentialamounts of pearlite. A lead-tin bath or a salt bath may be used, and thecasting is maintained therein for a certain time dependent on thetemperature to secure an isothermal transformation of the supercooledaustenite. This 'bainite reaction is not fully known, but a needle-likeor acicular structure is obtained, which is to some extent similar tomartensite it formed at a relatively low temperature and which is anintermediate stage between pearlite and martensite. In order to obviateas far as possible the formation of cementite during this slowtransformation it is advisable to hold the casting at the hardeningtemperature a period of time which is 5 to 10 minutes shorter than'thatindicated above for quenching in water so that the austenite has a verylow content of combined carbon. A holding time of 6-10 minutes in caseof small-sized castings has thus been found sufficient. The structuremay contain bainite, troostite and some ferrite depending on the natureof the heat treat ing. If the material is quenched down to about 200-250C. some martensite may be formed, and this can in a separate step betransformed into sorbite by tempering. The time of holding the materialin the bath, which holding time determines the effectiveness of theisothermal transformation, is dependent on the temperature. Figure 5shows examples of the heat cycles. The curve F differs from curve C inFigure 4 in that the casting is liquid quenched only to 250 C. and heldat this temperature for about 30 minutes, after which the material isquenched in water to transform into martensite such residual austenite,which may remain. According to curve G the material is liquid quenchedto 400 C., held at this temperature for about 35 minutes and thereaftercooled in air. The time for holding the casting in the bath depends onthe temperature thereof and the holding time for hardening. Testing rodsof Iron A were thus reheated for hardening to 960 C. and held at thistemperature for 17 minutes, whereafter the castings were quenched todifferent temperatures and held for 10 hours at 250 C., 4 hours at 275C., 50 minutes at 310 C., 6 minutes at 350 C. and 3 minutes at 400 C.Figure 22 shows the structure of the material quenched at 250 C., Figure23 quenched at 310 C. and Figure 24 quenched at 400 C. A highest tensilestrength of 163,000 p. s. i. and an elongation of 1.5% were attained byquenching to 310 C., and other tests have given as a result anelongation of 6% in association with 'a tensile strength of 125,000 p.s. i.

After the material has been water quenched according to Figure 4 it istempered or drawn at a period of time dependent on the sectional size ofthe casting to obtain tempered martensite. The diagram in Figure 6 showssome preferred heating cycles. The castings are placed in an ovenpreheated to 400 C. and removed after a certain time to be cooled in theair. The curves H, K and L indicate as examples the cycles for castingshaving a section size up to 25 mm., between 25-50 mm., and between50-100 mm. respectively, in which cases the total heating times are 40,60 and minutes respectively. The temperature should be above 250 C. andbelow 550 C., preferably between 350-450 C., whereby a hardness ofbetween 240 and 500 Brinell is obtained, but the preferred hardness isbetween 350-450 Brinell. In dependence of the temperature more or lesssorbite, troostite and similar tempering structures are formed. The bestcombination of tensile strength, hardness, elongation and wearresistance has been obtained by drawing at about 400 C. Figure 7 revealsthat the tensile strength sharply varies in dependence of the temperingtemperature. The

curves shown by full lines relate to the Iron A, and it is remarkablethat the curve TS for the tensile strength has a sharp maximum point at400 C. corresponding to 183,000 p. s. i., whereas 350 C. corresponds to121,000 p. s. i. and 450 C. to 126,000 p. s. i. The dotted curve showsthe result of other similar tests with a maximum at 400 C. of 200,000 p.s. i. The curve S representing the elongation shows a maximum of 2.0% atthe same temperature 400 C. and 1.3% at 350 C. and 450 C. The hardnessvaries as shown by the curve HB and is at 400 C. about 400 Brinell. Themicrophotogrnph in Figure 15 shows the mainly sorbitic structure of theIron A hardened from 900 C., tempered at 400 C. and representing themaximum point of curve TS. In commercial production it has been foundthat the elongation as an overage for the products obtained from 82consecutive charges was 1.85%.

It has been pointed out above that the holding time at the hardeningtemperature is of a vital importance for the properties of the finishedproduct, and a series of tests have been made for illustrating byphotographs the structure of the iron at varying holding times. For thispurpose test bars of the Iron A were used, and the structure of thisiron as cast is shown in Figure and as annealed in Figure 11. Figures12, 14, 16, 18 and 20 show the microstructure of the rods held at 900 C.for 8, 18, 25, 45 and 120 minutes respectively and thereafter quenchedin water. Figures 13, 15, 17, 19 and 21 correspond each to the precedingfigure and show the structure after tempering at 400 C. for 40 minutes.The composition of the iron and the heat treatment with a holding timeof 8, 18 and 25 minutes are thus in accordance with the invention.Figures 12-15 illustrate a material with temper carbon of normal form,size and distribution and without any cracks. Figure 16 reveals,however, that a fine fissure extends between difierent temper carbonparticles, although this does not reduce the tensile strengthessentially. In Figures 18 and 19 fissures pass across the wholephotograph, and the holding time 45 minutes has thus been too long sothat the carbon content of the austenite has exceeded allowable valuesand caused free cementite and cracks. Figures 20 and 21 reveal that mosttemper carbon has been dissolved under formation of cementite, and fromthe residues of the temper carbon severe fissures extend towards othercarbon centres. This solution of the temper carbon as shown in Figures20 and 21 and several other photographs from similar tests may serve asa support of my assumption that the point representing the carboncontent of the austenite in Figure 9 can pass along the temperature line900 C. into the field between the points V1 and U1 and possibly beyondU1.

The test rods of Iron A as annealed in accordance with Figures 3 and 11had a tensile strength of 46,500 p. s. i., an elongation of 15.4% and ahardness of 103 Brinell. The rods were then put into an oven preheatedto 900 C., and 18 minutes thereafter the rods were removed and droppedinto pouring water of C. They were tempered at 400 C. during 40 minutesand then tested. The result appears in Figure 7 which shows a tensilestrength of 183,000 p. s. i., an elongation of 2.0% and a hardness ofabout 400 Brinell. This material represents a normal product which ismade according to the invention and which is to be compared with thematerials produced and described below in which certain compositionelements have been increased.

Thus the deleterious effect of phosphorus has been pointed out abovewith reference to Figure 1, and in order to show the structure of aniron with too high amount of phosphorus test rods were cast from thesame charge as the rods Iron A but with a small addition of phosphorus.The composition as analyzed was 2.36% C, 0.89% Si, 0.19% Mn, 0.035% Sand 0.126% P. The castings as annealed had an average tensile strengthof 49,000 p. s. i., an elongation of 18.3%, a hardness of 88 Brinell andthus very desired properties. Five rods were a 12 after annealinghardened and tempered in the same way as the Iron A, and the tensilestrengths obtained were (1) 83,000, (2) 105,000, (3) 89,000, (4) 15,800and (5) 31,500 p. s. i. The hardness was 388 Brinell. The samples (4)and (5) had cracks extending almost through the whole rod, whereassamples (1), (2) and (3) had only intercrystalline fissures ormicrocracks of a considerable length. Although the rods were cast andheat treated in quite the same way the material was very uneven andunreliable. Figure 25 illustrates the microcracks in the structure ofsample (3) and Figure 26 the cracks in sample (5). It is obvious thatsuch a material cannot be commercially used, and that a phosphoruscontent as high as 0.126% considerably impairs the finished product.

In order to further show the eifect of a high content of manganese testrods were cast and heat treated as in the preceding case with theexception that the phosphorus content was the same as in Iron A and themanganese content was raised from 0.19% to 0.28%. The material asannealed showed a normal average tensile strength of 48,000 p. s. i., ahardness of 88 Brinell and an elongation reduced to 12.7% as a result ofthe higher manganese content. Five rods were hardened and tempered inthe same way as the Iron A and tested. The hardness was 388 Brinell, theaverage elongation was reduced to 1.1% and the tensile strength variedbetween 137,000 p. s. i. and 149,000 p. s. i. with an average of 145,000p. s. i. The slight increase of Mn from 0.19% in Iron A to 0.28% resultsthus in a lowering of the tensile strength from 183,000 p. s. i. to145,000 p. s. i. and the elongation from 2.0% to 1.1% probably dependingon the formtion of complex carbides of Fe, Si and Mn. Although theseproperties represent an improve ment compared with known products ofmalleable iron the tests show the impairment of the material byincreasing the manganese content. Figure 27 is a microphotograph of therod having the lowest tensile strength 137,000 p. s. i., and it appearsthat the structure is rather coarse and a crack is formed and partlyvisible at the periphery.

Tests have also been conducted to show the properties and structure ofthe material containing spheroidal carbon particles or nodules obtainedby addition of Mg, Ce, Ca or Li. For casting test rods a melt wasprepared, but as the material is particularly intended for use in heavycastings the contents of C and Si were slightly increased above thenormal values as in Iron A. The analysis showed 2.62% C, 1.14% Si, 0.19%Mn, 0.045% P and 0.024% S. The content of S is of special importance inthis material because it has been found that less than about 0.02% Swill result in a material having only spheroids or nodules and no tempercarbon. The proportion between these carbon particles can thus beregulated by the content of sulphur. In connection with casting the rods0.64% of a mixture of alloys was added which alloys contained Mg, Cu,Si, Fe and Ca. A completely white casting was obtained, which wassubjected to the same anneal process as Iron A. This material had atensile strength of 50,000 p. s. i., an elongation of 16.1% and ahardness of Brinell. The structure thereof is shown in Figure 28 inwhich a dense or spheroidal carbon particle and two temper carbon nestsare visible. After hardening and tempering of five rods in the same wayas Iron A the material had a hardness of 392 Brinell, an average tensilestrength of 176,000 p. s. i. and an average elongation of 1.9%. Figure29 illustrates the structure and shows a spheroid and a temper carbonnest. As a consequence of the high contents of C and Si the tensilestrength has been somewhat reduced, but by restoring them to normalvalues in connection with casting heavier sizes the tensile strengthwill be improved. It is obvious that also this material can be subjectedto isothermal transformation.

When following the above teachings it is possible to vary thecomposition and the heat treatment so as to obtain a productcorresponding to the properties as desired. The different steps can bewell controlled and a

1. THE METHOD OF FORMING AN IRON BODY, COMPRISING ANNEALING A WHITE IRONCASTING FREE OF FLAKE GRAPHITE HAVING A COMPOSITION OF 1.8 TO 3.2%CARBON, 0.5 TO 1.6% SILICON, THE TOTAL AMOUNT OF CARBON AND SILICONBEING LESS THAN ABOUT 3.8%, LESS THAN 0.1% SULFUR, 0.05 TO 0.45%MANGANESE, OF WHICH LESS THAN ABOUT 0.20% IS DISSOLVED MANGANESE, LESSTHAN 0.1% PHOSPHORUS AND THE REMAINDER SUBSTANTIALLY IRON, BY RAISINGTHE TEMPERATURE TO A RANGE BETWEEN 875%C. AND 1050*C. AND HOLDING THECASTING AT THIS TEMPERATURE UNTIL SUBSTANTIALLY ALL OF THE CEMENTITE ISDISSOLVED INTO AUSTENITE AND FREE CARBON PARTICLES OF SUBSTANTIALLYROUNDED FORM AND SLOWLY LOWERING THE TEMPERATURE TO AT LEAST BELOW750*C. TO OBTAIN A MATRIX SUBSTANTIALLY FREE OF CEMENTITE AND COMPRISINGFERRITE AND TEMPER CARBON, AND THEN HARDENING THE CASTING BY